1. Field of the Invention
The invention relates to a window interface layer of a light-emitting diode, and more particularly to a p-GaInP interface layer interposed between a p-GaP window layer and a p-AlGaInP layer for enhancing the film quality and the luminous efficiency as well as improving the electric property.
2. Description of the Related Art
With the rapid development of the Metal-Organic Chemical Vapor Deposition (MOCVD), the MOCVD process has been widely applied to the production of AlGaInP material with excellent crystallization. Accordingly, it is suitable for the mass production of light-emitting diodes with high luminance.
The conventional AlGaInP light-emitting diode is formed on an n-type GaAs substrate with a double heterostructure (DH). The double heterostructure includes a p-AlGaInP and an n-type AlGaInP cladding layer at a top and a bottom side while a luminous active layer is interposed between the cladding layers. In order to obtain a high luminous efficiency, the current spreading in the p-layer plays a very important role.
In order to ensure the current spreading for a high quality AlGaInP light-emitting diode, a thick window layer is provided atop the light-emitting diode. This material has to be transparent with respect to the wavelength of emitted light. Meanwhile, it possesses a high hole mobility and a high concentration of carrier density. The transparency depends on the wavelength and the window layer material. For this purpose, AlGaAs and GaP are two of the most frequently employed materials. The reason of using the AlGaAs is because the p-AlGaAs can be doped with an extremely high impurities while the hole mobility can be still maintained at a high level. Meanwhile, AlGaAs can be easily deposited on the GaAs substrate by use of the MOCVD process.
In addition, GaP is also an excellent choice. The p-GaP can also be doped with a high concentration of p-impurities. Meanwhile, it is transparent to red, yellow and green light. However, GaP and GaAs have a lattice mismatch of 3.6%.
Recently, a few of improved structures and techniques have been disclosed for enhancing the efficiency of light-emitting diodes. As shown in FIG. 1, a light-emitting diode employed by Hewlett-Packard (HP) includes a thick GaP window layer for the purpose of better current spreading and light extraction. The structure in FIG. 2 is disclosed by Toshiba and employs p-AlGaAs as window layer. Meanwhile, a current-blocking layer and a distributed Bragg reflector layer are added thereto. The structure shown in FIG. 3 employs indium-tin oxide (ITO) as current spreading layer. Besides, a distributed Bragg reflector layer is also applied thereto.
However, the use of the material of GaAsP or GaP in the window layer results in lattice mismatch with the substrate, thereby affecting the operational life time of the components. Moreover, the window layer made of ITO, SnO, InO, or the like has a transparency approaching to 90% in the range of visible light. Meanwhile, it has a resistivity of about 3×10−4Ω-cm, approximately one percent of the resistivity of p-type GaP. However, the light emitted from the side of the die can't be effectively utilized in the case of an optimal thickness ranging from 0.1 to 5 μm, thereby limiting the luminous efficiency of the light-emitting diodes.
In order to resolve the problem of poor reliability, U.S. Pat. No. 5,359,209 teaches a twin window layer structure shown in FIG. 4. The window layer contains a first window layer made of GaAs with the band gap smaller than that of the active layer and a second window layer made of GaP with the band gap higher than that of the active layer. This structure can stabilize dislocation defects the forward voltage Vf due to the defects at the interface. However, the window layer absorbs light due to the fact that GaAs used in the window layer has a band gap less than the active layer. Accordingly, the LED's brightness is much affected. Thus, the twin window layer consisting of GaAs and GaP disclosed in U.S. Pat. No. 5,359,209 still leaves some room for improvement.